1. Field of the Invention
The invention relates to a process for the preparation of 3-acryloyloxypropylalkoxysilanes.
2. Description of the Background
3-Acryloyloxypropylalkoxysilanes, especially the 3-methacryloyloxypropylalkoxysilanes, are among the organic silanes which are used on a large scale in industry. Since their structure includes both a hydrolysis-sensitive alkoxysilyl group, which can enter into a stable bond with inorganic materials, and a double bond which is active towards the organic reactants, the 3-acryloyloxypropylalkoxysilanes lend themselves to a wide variety of possible applications, for example, as coupling agents. 3-Methacryloyloxypropyltrimethoxysilane is frequently also used for modifying surfaces in the glass fibre industry.
It is known that various platinum compounds can be employed as catalysts for direct reaction of hydridosilanes with the allyl esters of acrylic and methacrylic acid. The catalyst system most frequently employed in industry for this hydrosilylation reaction is homogeneous and comprises hexachloroplatinic acid in acetone or isopropanol. The use of this high-chlorine platinum(IV) compound for the preparation of 3-methacryloyloxy- and 3-acryloyloxypropylalkoxysilanes is described in EP-A 0 277 023, EP-B 0 247 501 and EP-A 0 472 438.
EP-B 0 247 501 discloses a number of platinum(II) complexes as possible alternative catalysts of hexachloroplatinic acid, dichlorobis(acetonitrile)platinum(II), dichlorobis(ethylene)platinum(II), cis-dichlorobis(triphenylphosphino)platinum(II) and a platinum(O) complex-tetrakis(triphenylphosphino)platinum(O).
A further chlorine-containing platinum(II) catalyst, dichloro(1,5-cyclooctadiene)platinum(II), and another platinum(O) catalyst, 1,3-divinyltetramethyldlsiloxane/ platinum complex in toluene, are disclosed in EP-A 0 472 438 for the direct reaction of allyl methacrylates with trialkoxysilanes.
The yields disclosed in the above-mentioned prior art for the preparation of 3-acryloyloxy- and 3-methacryloyloxy-propylalkoxysilanes, using the chlorine-containing platinum catalysts mentioned and taking into account the particular reaction regime, are in the range from 75 to 88%. The prior art processes require laborious working up of the crude product formed.
The use of the prior art catalysts, moreover, because of their high chlorine content, which may amount to as much as 50% or more of their molecular weight, must be considered in relationship to the environment. Furthermore, these chlorine-containing catalyst systems must be employed as highly dilute solutions in acetone or isopropanol.
A considerable problem in all of the prior art processes is the unwanted tendency of the final products to polymerize. According to the teaching of EP-A 0 472 438, the tendency of the products to polymerize can be prevented by addition of sterically hindered phenols, amines, amides and aminophenols to the reaction mixture. In most cases the commercially available polymerization inhibitors are employed in quantities of from 1 to 5,000 equivalents of the platinum catalyst. Furthermore, it is possible, by continuous introduction of a gas containing molecular oxygen, to suppress polymerization. A gas mixture of this kind normally contains from around 0.1 to 20% by volume of oxygen.
From the literature it is known that the majority of hydrosilylation reactions proceed via a catalytically active species with platinum in oxidation states (II) and (IV). Even tetrakis(triphenylphosphine)platinum(O) has not acquired any great economic importance because of its instability towards oxygen, resulting in a relatively long induction period in the hydrosilylation, and because of its markedly lower activity and chemoselectivity in the reaction of allyl compounds with hydridosilanes.
The chlorine-free catalyst disclosed in EP-A 0 472 438-1,3-divinyltetramethylsilane/platinum complex, which contains platinum in oxidation state (0), can likewise be employed only as a dilute solution and, furthermore, is stable only for a limited time. Its activity and selectivity with regard to allyl compounds and acrylic compounds, furthermore, is inadequate for industrial purposes. A need therefore continue to exist for an economically more favorable catalytic process for preparing 3-acryloyloxypropylalkoxysilanes with minimal environmental impact.